2006
Chem. Mater. 1998, 10, 2006-2012
Trivalent Rare Earth Ion Conduction in the Rare Earth Tungstates with the Sc2(WO4)3-Type Structure Nobuhito Imanaka, Yasuyuki Kobayashi, Kazuyasu Fujiwara, Takuya Asano, Yusuke Okazaki, and Gin-ya Adachi* Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received March 12, 1998. Revised Manuscript Received April 27, 1998
To realize a trivalent ion conduction in solids, the Sc2(WO4)3-type structure was chosen on the basis of the mobile trivalent ions and the structure which reduces the electrostatic interaction between the framework and the mobile trivalent ionic species as much as possible. The typical conductivity of the rare earth tungstates R2(WO4)3 (R ) Sc, Y, and Er-Lu) with the Sc2(WO4)3-type structure was found to be on the order of 10-5 S cm-1 at 600 °C. Among the rare earth tungstates, Sc2(WO4)3 (σ600°C ) 6.5 × 10-5 S cm-1, Ea ) 44.1 kJ mol-1) was found to be the most suitable size for the ionic conduction with regard to the relation between the mobile ion radius and the lattice size. The rare earth ion conducting characteristics were investigated by means of the rare earth concentration cell measurements and dc electrolyses. The electromotive force measurements with the Sc-Y binary alloy and the yttrium tungsten bronze as the electrodes strongly suggest the possibility of the trivalent ion conduction of rare earths such as Sc3+ and Y3+. Furthermore, by the dc electrolysis, the mobile species was clarified to be the rare earth ions Sc3+ and Y3+ in the rare earth tungstates with the Sc2(WO4)3-type structure.
Introduction The ionic conducting species in solid electrolytes is limited to, in most cases, only one species without any electron or hole conduction. Solid electrolytes have peculiar characteristics such as transporting specific ions macroscopically with accompanying electric charge. The current great interest in the solid electrolyte field has been devoted to developing new electrolytes and to seek for their further application. In general, the ionic mobility in solid electrolytes strongly depends on the valency and ionic size of the mobile ion. The mono- and divalent cationic conductors have the advantage of high ionic conductivity with an associated low activation energy. In contrast, trivalent cations have been regarded to be an extremely poor mobile species for the ionic migration, due to the high electrostatic interaction of the highly charged species with the surrounding framework structure of anions such as O2-. Therefore, only a few papers have reported the possibility of trivalent ion conduction; e.g. Ln3+β′′-alumina,1-8 β-LaNb3O9,9 LaAl11O18,10 and * To whom all correspondence should be addressed. (1) Dunn, B.; Farrington, G. C. Solid State Ionics 1983, 9 & 10, 223. (2) Carrillo-Cabrera, W.; Thomas, J. O.; Farrington, G. C. Solid State Ionics 1983, 9 & 10, 245. (3) Ghosal, B.; Mangle, E. A.; Topp, M. R.; Dunn, B.; Farrington, G. C. Solid State Ionics 1983, 9 & 10, 273. (4) Farrington, G. C.; Dunn, B.; Thomas, J. O. Appl. Phys. 1983, A32, 159. (5) Dedecke, T.; Ko¨hler, J.; Tietz, F.; Urland, W. Eur. J. Solid State Inorg. Chem. 1996, 33, 185. (6) Ko¨hler, J.; Urland, W.Solid State Ionics 1996, 86-88, 93. (7) Ko¨hler, J.; Balzer-Jo¨llenbeck, G.; Urland, W. J. Solid State Chem. 1996, 122, 315.
LaAl12O18N.10,11 These papers, however, have only indicated a probability of trivalent cation conduction, and any trivalent ionic conduction in the solids were not directly and quantitatively demonstrated at all. One approach to realize the trivalent ion conducting solid electrolytes is to select a suitable structure from the stand point of a mobile trivalent ion in solids. In addition, the mobile ion candidates should be able to hold trivalency without varying the valency and have a relatively smaller ionic radius. Compounds with a larger tunnel size, which effectively works to reduce the electrostatic interaction between the mobile ions and framework of the structure as much as possible, possess the proper framework structure. The structure chosen as the most suitable one for the trivalent ion migration was the Sc2(WO4)3-type one,12-16 and aluminum and rare earth ions were selected as the mobile ion candidates. The hexavalent tungsten ion, W6+, which is one of the constituent ions, reduces the interaction between a mobile trivalent ion and constituent oxide ions, since (8) Ko¨hler, J.; Urland, W. Angew. Chem., Int. Ed. Engl. 1997, 36, 85. (9) George, A. M.; Virkar, A. N. J. Phys. Chem. Solids. 1988, 49, 743. (10) Warner, T. E.; Fray, D. J.; Davies, A. Solid State Ionics 1996, 92, 99. (11) Warner, T. E.; Fray, D. J.; Davies, A. J. Mater. Sci. 1996, 32, 279. (12) Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1966, 45, 2745. (13) Nassau, K.; Levinstein, H. J.; Loiacono, G. M. J. Phys. Chem. Solids. 1965, 26, 1805. (14) Borchardt, H. J. J. Chem. Phys. 1963, 39, 504. (15) Nassau, K.; Shiever, J. W.; Keve, E. T. J. Solid State Chem. 1971, 3, 411. (16) Rode, E. Y.; Lysanova, G. V.; Kuznetsov, V. G.; Gokhman, L. Z. Russ. J. Inorg. Chem. 1968, 13, 678.
S0897-4756(98)00157-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/09/1998
Trivalent Rare Earth Ion Conduction
O2- ions in the tungstates are strongly bonded to W6+ ions, which hold twice the valency of the trivalent mobile ion. As a result, the trivalent ion is left to migrate in the tungstates smoothly. Scandium tungstate is isomorphous with the tungstates of trivalent ions such as Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, Lu, In, and Al. A comprehensive study of the electrical transport properties for heavy rare earth tungstates with the Sc2(WO4)3-type structure has been done by Verma et al.17 They concluded that the contribution to the electrical conductivity of the rare earth tungstates is due to the migration of ions or electrons or both. At higher temperatures, the electronic conduction dominates over the ionic conduction, but at lower temperatures, the ionic conduction appears. The mobile ion, however, has not been identified. Recently, we have directly demonstrated conduction of a trivalent ion such as Al3+, Sc3+, Y3+, and Er3+ in the tungstate with the Sc2(WO4)3-type structure.18-20 The aim of the present study is to clarify the individual trivalent species dependencies on both the trivalent ion conducting characteristics and the structure of the trivalent tungstates with the Sc2(WO4)3-type structure. The mobile trivalent rare earth ion was identified by using both thermodynamic and kinetic measurements, that is, the rare earth concentration cell, polarization, and electrolysis. Experimental Section Sample Preparation. Rare earth tungstates, R2(WO4)3 (R ) Sc, Y, Er, Tm, Yb, and Lu), were prepared by a conventional solid-state reaction. A stoichiometric amount of R2O3 (purity 99.9%) and WO3 (purity 99.9%) was mixed in a mortar and calcined on a platinum boat at 1000 °C for 12 h in air. The calcined powder was ground and reheated at 1200 °C for 12 h in air. The resulting powder was made into pellets (10 mm in diameter) and sintered at 1300-1400 °C for 12 h in air. In the case of Y and heavy rare earths, Er-Lu, the sample powder was dried in a vacuum at 150 °C before sintering, because they are so hygroscopic at ambient temperature. The procedure for the dense pellet preparation of Al2(WO4)3 is the same as described previously.20 The samples were characterized by X-ray powder diffractometer and high-temperature X-ray powder diffractometer using Cu KR radiation (M18XHF, Mac Science). The XRD data were collected by a step-scanning method in the 2θ range from 10° to 120° with a step width of 0.04° and a scan time of 4 s. The XRD patterns obtained were analyzed using the Rietveld refinement program RIETAN-94.21 Conducting Property Measurements. Electrical conductivity was measured by an ac and a dc method, using the sintered ceramic pellet with two platinum electrodes in the temperature range from 200 to 900 °C. The ac conductivity measurements were carried out by an ac complex impedance method in the frequency range from 20 Hz to 1 MHz by using Hewlett-Packard precision LCR meter (8284A). The dc conductivity was measured with two platinum electrodes as an ion-blocking electrode. The dc voltage (1 V) was applied between the Pt electrodes sandwiching the sample, and the current passed through the sample was monitored as a function of time in oxygen (PO2 ) 1 × 105 Pa) and helium (PO2 ) 4 Pa) atmosphere. (17) Verma, B. K.; Lal, H. B. Mater. Res. Bull. 1981, 16, 1579. (18) Imanaka, N.; Kobayashi, Y.; Adachi, G. Chem. Lett. 1995, 433. (19) Imanaka, N.; Adachi, G. J. Alloys Compd. 1997, 250, 492. (20) Kobayashi, Y.; Egawa, T.; Tamura, S.; Imanaka, N.; Adachi, G. Chem. Mater. 1997, 9, 1649. (21) Izumi, F. The Rietveld Method; Oxford University Press: Oxford, 1993; Chapter 13.
Chem. Mater., Vol. 10, No. 7, 1998 2007 To determine the ionic transference number, ti, of the sample, the oxygen-air concentration cell measurements as described in eq 1 were performed in the temperature range between 600 and 900 °C.
Pt, O2 (PO2 ) 1 × 105 Pa)|R2(WO4)3|air (PO2 ) 2.1 × 104 Pa), Pt (1) Furthermore, a constant current was passed through the concentration cell, and the terminal voltage was monitored to clarify whether the oxide ion is the conducting species or not. Rare Earth Concentration Cell. The trivalent ion conduction was investigated by rare earth concentration cell measurements using scandium-yttrium binary alloys and yttrium tungsten bronze as the electrodes. Sc-Y Binary Alloy. Two types of Sc-Y binary alloys with composition of 10 and 90 mol % Y , Sc90Y10 and Sc10Y90, were used as electrodes. The alloys were prepared by arc melting the appropriate mixture of scandium and yttrium metals (purity 99.9%) under an argon atmosphere. The arc-melted alloys were remelted at least four times to homogenize the composition. The alloys were annealed at 1000 °C for 50 h in a purified helium atmosphere (PO2 ) 10-9 Pa) passing through the liquid nitrogen trap. The alloys were cut into thin tablets to use as the electrode (10 mm in diameter). The electromotive force (emf) measurements were carried out with Sc2(WO4)3 or Y2(WO4)3 as a solid electrolyte by constructing the following cell in a purified helium gas atmosphere at 300 °C:
electrode(A) Sc10Y90|Sc2(WO4)3|Sc90Y10 electrode(B)
(2)
electrode(A) Sc10Y90|Y2(WO4)3|Sc90Y10 electrode(B)
(3)
The open-circuit voltages were measured using a digital electrometer (R8240, Advantest) with an input impedance of 1013 Ω. Yttrium Tungsten Bronze. Yttrium oxide, tungsten trioxide, and tungsten metal were mixed in the appropriate molar ratio.22,23 The powder was pelletized and sealed into a quartz tube under 6.7 × 10-2 Pa and was heated for 50 h at 1100 °C. To guarantee its homogeneity, the products were regrounded and reheated several times. All samples were cooled rapidly down to room temperature. The lattice constant of yttrium tungsten bronze, YxWO3 (x ) 0.09-0.15), was determined by X-ray powder diffraction using high-purity Si powder (purity 99.999%) as an internal standard. The emf measurements were carried out by constructing the following electrochemical cell with Y2(WO4)3:
Y0.12WO3|Y2(WO4)3|YxWO3 (x ) 0.09, 0.10, and 0.12)
(4)
Cubic bronzes, YxWO3 (x ) 0.09-0.12), were used as the electrode, while Y0.12WO3 was fixed for the reference electrode. The cell was heated at 300 °C under a helium gas atmosphere purified by passing through the liquid nitrogen trap. Electrolysis. The dc electrolysis was performed with two platinum plates as ion-blocking electrodes by applying a direct voltage of 10 V for 250-300 h at 1000 °C in air. After the electrolysis, both the cathodic and anodic surfaces of the samples were analyzed by X-ray microarea diffractometer, scanning electron microscope (SEM, S-800, Hitachi), and electron probe microanalysis (EPMA-1500, Shimadzu). The electrolyzed samples were mounted in a epoxy resin, cut so that the cross-sectional EPMA line analysis could be done from the anode to cathode direction, and then polished with a diamond paste of average diameter 1 µm. (22) Ostertag, W. Inorg. Chem. 1966, 5, 758. (23) Broyde, B. Inorg. Chem. 1967, 6, 1588.
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Figure 2. The trivalent ionic radius dependence of the unit cell volume for M2(WO4)3. Table 1. Lattice Parameters and Unit Cell Volume of the Trivalent Tungstates, M2(WO4)3 ionic M3+ radiusa/nm a-axis/nm b-axis/nm c-axis/nm cell vol/nm3
Figure 1. XRD patterns of the trivalent tungstates with the Sc2(WO4)3-type structure: (a) Al2(WO4)3, (b) Sc2(WO4)3, and (c) Y2(WO4)3.
Results and Discussion Sc2(WO4)3-Type Structure. The Sc2(WO4)3-type structure is orthorhombic with the space group Pbcn and is built up with a three-dimensional skeletonal framework of corner-linked ScO6 octahedra and WO4 tetrahedra.12,24 Each ScO6 octahedron is connected to six WO4 tetrahedra, while each WO4 tetrahedron is connected to four ScO6 octahedra. The basic unit of the network consists of two octahedra and three tetrahedra, [Sc2W3O18]. These units are linked together via vertexes and connected to form a layer along the c-axis. This structure is similar to the frame network of lithium scandium phosphate,25 γ-Li3Sc2(PO4)3, which has been already reported to be a high lithium ionic conductor. Lithium ions in Li3Sc2(PO4)3 lie in three crystallographic positions and conduct through the opening of the framework consisting of ScO6 octahedra and PO4 tetrahedra. Furthermore, the Sc2(WO4)3-type structure is closely related to that of the NASICON-type. The framework structure of NASICON is also composed of the unit [X2Z3O18]n-; each X atom is surrounded with six oxygens situated at vertexes of six ZO4 tetrahedra, and each Z atom is surrounded by four oxygens situated at vertexes of four XO6 octahedra.24 Na+ ions are capable of conducting through the opening path composed of the three-dimensional framework structure. The main difference between Sc2(WO4)3 and the NASICON-type structure is only the arrangement of the basic unit. Therefore, the Sc2(WO4)3 structure is expected to possess the unusual feature of a large tunnel size. The typical X-ray powder diffraction patterns for the samples R2(WO4)3 (R ) Al, Sc, and Y) at room temperature are shown in Figure 1. The X-ray diffraction (24) Piffard, Y.; Verbaere, A.; Kinoshita. M. J. Solid State Chem. 1987, 71, 121. (25) Bykov, A. B.; Chipkin, A. P.; Demyanets, L. N.; Doromin, S. N.; Genkina, E. A.; Ivanov-Shits, A. K.; Kondratyuk, I. P.; Maksomov, B. A.; Melnikov, O. K.; Muradyan, L. N.; Simonov, V. I.; Timofeeva, V. A. Solid State Ionics 1990, 38, 31.
Al Sc Lu Yb Tm Er Y a
0.0675 0.0885 0.1001 0.1008 0.1020 0.1030 0.1040
1.2599 1.3328 1.3781 1.3806 1.3849 1.3905 1.3954
0.9061 0.9586 0.9875 0.9897 0.9924 0.9959 0.9988
0.9141 0.9677 0.9967 0.9988 1.0017 1.0049 1.0077
1.0435 1.2346 1.3564 1.3648 1.3767 1.3916 1.4045
Six coordinated.29
analyses for the compounds of Y and Er-Lu were done in a vacuum at room temperature after drying at 150 °C in a vacuum, since they are so hygroscopic.13 All tungstates were found to be a single phase with an orthorhombic symmetry (space group Pbcn), and the XRD patterns of the sample were shifted toward low 2θ with increasing trivalent ionic radius26 in the tungstates. Figure 2 presents the trivalent ionic radius dependencies on the unit cell volume for the tungstate system. The trivalent ionic radius, the lattice parameters, and the unit cell volume refined by the Rietveld method for the trivalent tungstates are summarized in Table 1. The lattice parameters and the cell volume increased upon increasing the trivalent ionic radius. The expansion rate of each lattice parameter toward the trivalent ionic radius was equal and isotropic expansion was observed. Conducting Properties. The typical complex impedance plot (Cole-Cole plot) for the samples was composed of two kind of components: a semicircle and a line with a 45° slope to the abscissa in the higher and lower frequency region, respectively. In the case of using two types of electrode, Pt or Au, the different behavior was observed in the slope of the line in the lower frequency region. In contrast, no change in the semicircle was recognized. From this result, it becomes clear that the conductivity obtained by an intersecting point of the semicircle in the higher frequency region is identified with the total conductivity of the sintered pellet containing the grain bulk and the grain boundary, similar to the results reported for Al2(WO4)3.20 The temperature dependencies of the typical electrical conductivity for M2(WO4)3 (M ) Al, Sc, and Y) are depicted in Figure 3. The temperature dependence of (26) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
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Figure 5. The ac conductivity (O) and the time dependence of the dc conductivity for Sc2(WO4)3 in air. Figure 3. The temperature dependence of the electrical conductivity for M2(WO4)3 (M ) Al, Sc, and Y).
Figure 4. The trivalent ionic radius dependence of the electrical conductivity at 600 °C and the activation energy for M2(WO4)3.
the ionic conductivity is usually expressed by the following Arrhenius equation:
σT ) σ0 exp(-Ea/RT)
(5)
where Ea, σ, T, σ0, and R denote the activation energy, the electrical conductivity, the absolute temperature, the preexponential factor, and the gas constant, respectively. The activation energy (Ea) was calculated from eq 5 in the temperature range from 400 to 600 °C, where the relationship between σT and 1/T is a straight line. The electrical conductivity at 600 °C and the Ea variation for the tungstates as a function of the trivalent ionic radius is plotted in Figure 4. The conducting properties depend on a kind of trivalent ion in the tungstates, and the most suitable mobile trivalent ion size exists in the trivalent tungstate series with the Sc2(WO4)3-type structure. It becomes clear from the figure that Sc2(WO4)3 exhibits the highest conductivity, 6.5 × 10-5 S cm-1 at 600 °C, and the lowest activation energy, 44.1 kJ mol-1, among the tungstates with the Sc2(WO4)3-type structure. In the case of the trivalent ions larger than the most suitable trivalent ion, Sc3+, the
electrical conductivity decreased and the activation energy enhanced upon increasing the ionic radius. This is because the increasing ratio of the ionic size is larger than that of the lattice size. In contrast, for the smallest ion, Al3+, the conductivity was the lowest and the activation energy was the highest in the tungstate series. The extremely poor migration of Al3+ in the Sc2(WO4)3-type structure is due to the high electrostatic interaction caused by its low polarizability, with surrounding anions composing the skeleton structure. To investigate the electrical conducting properties of the rare earth tungstates, the dc conductivity was measured as a function of time and compared with the ac one. Figure 5 shows the time dependencies of the dc conductivity at the various temperatures in air for Sc2(WO4)3, which shows the highest electrical conductivity among the trivalent tungstates, as well as the data of the ac conductivity. The dc conductivities were found to decrease abruptly compared with the ac conductivities and approached a steady current. The dc conductivities were 2-3 orders of magnitudes lower than ac ones in the whole temperature region measured. These polarization phenomena suggest that the main conducting species is neither an electron nor a hole but only an ion and that the ionic transference number is expected to be over 0.99 for scandium tungstate. To examine the possibility of O2- conduction, the dc conductivity was measured at 700 °C in two different atmospheres: O2 (PO2 ) 105 Pa) and He (PO2 ) 4 Pa). A clear polarization behavior, that the ratio of dc to ac conductivity (σdc/σac) abruptly decreased from unity, was observed in both atmospheres. If an O2- ion is the predominant mobile ionic species, a clear polarization would be observed in the helium atmosphere, while in the oxygen atmosphere the dc conductivity would be the same as that obtained by ac method and no polarization behavior would be observed, as described in ref 20. Having the same polarization in the two atmospheres, therefore, reveals that the oxide ion is eliminated as a candidate for the mobile ionic species.
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Figure 6. The temperature dependence of the electromotive force (emf) for Sc2(WO4)3 of the oxygen-air concentration cell. The straight line shows the theoretical emf.
Figure 7. The composition dependence of the cubic cell parameter, a0, for yttrium tungsten bronze, YxWO3 (x ) 0.090.15).
Furthermore, the ionic transference number, ti, was investigated by constructing the oxygen-air concentration cell and compared with the result obtained by the polarization measurements. The theoretical emf values of the cell were calculated from the following Nernst equation:
E)-
RT PO2(II) ln 4F PO2(I)
(6)
where F is the Faraday constant. The measured and theoretical emf values coincided well in the temperature range from 600 to 900 °C, as presented in Figure 6 and the ti was estimated to be unity and identical to the ti value of over 0.99 from the polarization measurements. In the case of the oxide ion conductor, it is possible to draw a constant current from the concentration cell. By drawing the current, the terminal voltage between the sample appreciably decreased and a steady current could not be derived. Therefore, it can be easily elucidated from this phenomenon that the oxide ion is not a conducting species in the rare earth tungstate. This also supports the result obtained from the polarization analysis described above. Rare Earth Concentration Cell. Sc-Y Binary Alloy. The scandium or yttrium concentration cell measurements were performed using Sc-Y alloy27 electrodes of two different compositions, Sc90Y10 and Sc10Y90, with Sc2(WO4)3 or Y2(WO4)3, which were considered to be a Sc3+ or a Y3+ conductor, respectively. In the case of the Sc3+ ion conductor, Sc3+ ions would be transported from the electrode B Sc90Y10 to the electrode A Sc10Y90, since the scandium activity at electrode B is higher than that at electrode A in the concentration cell (eq 2). On the other hand, in the cell in eq 3, Y3+ ions diffuse into the opposite direction of cell 2 and cell 3 generates the reversed emf toward cell 2. In the case of Sc2(WO4)3, the emf obtained with cell 2 was around 20 mV. In contrast to this, when Y2(WO4)3 was set for Sc2(WO4)3, the emf that appeared with cell 3 was about -20 mV. The emf outputs of the cell with Sc2(WO4)3 and Y2(WO4)3 were closely related to the difference in Sc and Y activity, respectively. This phenomena strongly suggest that the conducting species is neither an (27) Masslski, T. B. Binary Alloy Phase Diagrams; American Society of Metals: Metals Park, OH, 1986; p 2037.
Figure 8. The relationship between emf obtained by the yttrium concentration cell (eq 4) and ln(a0(x)/a0(0.12)) (x ) 0.09, 0.10, and 0.12).
electron nor a hole and that the mobile species is strongly expected to be the Sc3+ and Y3+ ion in Sc2(WO4)3 and Y2(WO4)3, respectively. Yttrium Tungsten Bronze. Tungsten bronzes containing the rare earth have been already reported22,23 and it was clarified that scandium does not form a tungsten bronze. Y and lanthanoid tungsten bronzes exist in a tetragonal form and a cubic perovskite-type structure at lower and higher concentration of the rare earth, respectively. In the case of yttrium, below x ) 0.09 a tetragonal form appears and above x ) 0.15 two additional phase, Y2(WO4)3 and WO2, are present. The yttrium tungsten bronzes YxWO3 (x ) 0.09-0.15) obtained were a blue-violet powder and were found to be a single cubic phase from XRD measurements. The cubic lattice parameter, a0, of the yttrium tungsten bronzes obtained linearly increased with the yttrium content when x is between 0.09 and 0.12, as shown in Figure 7. Above x ) 0.12, a0 was constant as the yttrium content increased. Therefore, a homogeneous cubic perovskite-type structure is found for YxWO3 when x is between 0.09 and 0.12. The yttrium concentration cell was constructed by using three types of tungsten bronzes, x ) 0.09, 0.10, and 0.12, which have a cubic phase. Y0.12WO3 was used as the reference electrode. The emf values for the cell with Y0.12WO3 | Y2(WO4)3 | Y0.09WO3, Y0.12WO3 | Y2(WO4)3 | Y0.10WO3, and Y0.12WO3 | Y2(WO4)3 | Y0.12WO3 system were steady at 92, 62, and 3 mV, respectively. Structural, electrical, and thermodynamic properties have been precisely investigated with respect to sodium
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Figure 9. SEM photographs of the (a) cathodic and (b) anodic surfaces of Sc2(WO4)3 after the electrolysis.
tungsten bronze. The tungsten bronze, MxWO3, shows the metallic conduction because of a mixed valency state of W5+/W6+, while it exhibits the ionic conduction due to Mn+ ion diffusion. Therefore, sodium tungsten bronze is applicable as a Na+ reversible electrode for measuring the conductivity of the Na+ ion conductor.28 The Na activity in the sodium tungsten bronze NaxWO3 has been measured using an electrochemical cell with Pyrex as a solid electrolyte and reported to be directly proportional to sodium concentration in the cubic phase region.29 The Y activity in yttrium tungsten bronze is not known yet. However, in the YxWO3 system, the yttrium activity is expected to be directly related to the yttrium concentration in a cubic bronze, YxWO3, with x between 0.09 and 0.12. Therefore, the relation between the emf vs lattice parameter was examined here
aY(x) a0(x) RT ln E)∝ ln 3F aY(0.12) a0(0.12)
(7)
where ao and aY are the lattice parameter and the yttrium activity in the yttrium tungsten bronzes, respectively. Figure 8 shows the relationship between the measured EMF and ln(a0(x)/a0(0.12)) (x ) 0.09, 0.10, and 0.12). The emf values obtained by the yttrium concentration cell (eq 4) were proportional to the logarithm of the cubic lattice parameter ratio of YxWO3 and were found to follow the Nernst equation (7). This indicates that Y2(WO4)3 is a Y3+ pure ionic conductor. Electrolysis. To directly determine the mobile ionic species in the rare earth tungstates, an electrolysis was carried out by sandwiching the tungstate between two platinum electrodes. Here, the tungstate that showed the highest conductivity was selected, Sc2(WO4)3. The (28) Whittingham, M. S.; Huggins, R. A. J. Chem Phys. 1971, 29, 429. (29) Ramanarayanan, T. A.; Worrell, W. L. J. Electrochem. Soc. 1974, 121, 1530.
Table 2. Scandium and Tungsten Content of the Bulk before the Electrolysis and Those of the Deposits on Cathodic and Anodic Bulk Surface for Sc2(WO4)3 after the Electrolysis content/atom % shape bulk cathode anode
needlelike platelike
Sc
W
39.7 84.3
61.9 15.7 100
SEM photographs of the cathodic and anodic bulk surfaces of Sc2(WO4)3 after the electrolysis were presented in Figure 9. Many needle-shaped deposits were observed on the cathodic surface (panel a). In contrast, the color of the anodic surface of the sample changed from white before the electrolysis to yellow after the electrolysis and plate-shaped deposits were observed by SEM (panel b). From EPMA measurements, it was found that the needle-shaped deposits contained scandium and tungsten and that the ratio of the scandium to tungsten element in the deposits considerably increased to about 6:1 compared with that of the tungstate bulk (2:3) before the electrolysis (see Table 2). On the other hand, the plate-shaped deposits that appeared on the anodic surface were found to contain only tungsten. Figure 10 shows the X-ray microarea diffraction patterns of the surface after and before the electrolysis for Sc2(WO4)3. The deposits on cathodic and anodic surface after the electrolysis were identified as Sc6WO12 and WO3, respectively. The ratio of composition of the deposits on cathodic surface was in exact agreement with that measured by EPMA. It was clearly shown that the element Sc was segregated on the cathodic surface after the electrolysis. The cross-sectional EPMA line analysis for Sc2(WO4)3 after the electrolysis is presented in Figure 11. The profile of Sc showed a large segregation of the element Sc at the cathodic surface of the electrolyte, while W was almost flat in the all region, except for at the
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Figure 11. Cross-sectional EPMA line analysis of scandium and tungsten element for the Sc2(WO4)3 pellet after the electrolysis.
Figure 10. X-ray microarea diffraction patterns of the (a) cathode and (b) anode deposits after the electrolysis, and (c) the bulk surface before the electrolysis for Sc2(WO4)3.
cathodic side where W decreased due to the deposition of scandium. These behaviors strongly suggest that trivalent Sc3+ ion migrates from anode to cathode and that the scandium segregation occurs on the cathodic surface, while tungsten oxide is left on the anodic surface. The same result was obtained for Y2(WO4)3 also. These results clearly indicate that the trivalent rare earth ions Sc3+ and Y3+ in the Sc2(WO4)3-type structure are the predominant mobile ionic species in the tungstates. Conclusion The Sc2(WO4)3-type structure was selected upon considering both the mobile trivalent ions and the structure which reduces as much as possible the elec-
trostatic interaction between the mobile trivalent ionic species and the framework of the structure. By the electromotive force measurements and by the dc electrolysis, the mobile ion species in the Sc2(WO4)3-type structure was clearly demonstrated to be the trivalent rare earth ions of Sc3+ and Y3+ in the rare earth tungstates. Acknowledgment. We thank Dr. K. Yamada for the EPMA measurements and Dr. K. Toda for the Rietveld analysis. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 09215223 on Priority Areas (No. 260), Nos. 06241106, 06241107, and 093065 from The Ministry of Education, Science, Sports and Culture. One of the authors (Y.K.) is the Fellowship member of the Japan Society for the Promotion of Science for Japanese Junior Scientists. This work was also supported by the “Research for the Future, Preparation and Application of Newly Designed Solid Electrolytes (JSPS-RFTF96P00102)” Program from the Japan Society for the Promotion of Science. CM980157E
